1. Field of Invention
The present invention relates to a universal method and apparatus for the determination of moisture content of material, manufactured or natural, such as cereal grain and seed, for example, from radio-frequency measurement of their dielectric properties independent of bulk density changes, structure (shape, dimensions, and surface characteristics), and composition of the material using a universal calibration algorithm that remains valid across instruments of different designs.
2. Description of the Related Art
Moisture content of natural or manufactured particulate and granular materials is crucial in many industries including mining, construction, pharmaceutical, food and agriculture. It is commonly used as a quality control factor and/or an indicator for the optimization of a given process. Methods for moisture content determination in natural and manufactured materials can be classified into two categories: direct methods and indirect methods. Direct methods rely on weight loss (oven drying method) or chemical titration (Karl-Fisher). Direct methods require off-line testing of a few samples. They are accurate and are usually used as references for the calibration of other methods. The major disadvantages are their destructive nature and the time they require. Therefore, they do not meet the needs of highly automated industries for which moisture content has to be determined in real time for large quantities of material. Indirect methods are based on the measurement of a property of the material that is directly correlated with moisture content. Nuclear radiation-, infrared- and dielectric-based sensors are the most commonly used indirect measurement techniques. They have the advantage of being nondestructive, contactless and, most importantly, provide a tool for on-line continuous measurement of moisture content. Moreover, with better sampling and averaging over the entire material volume, they give a better estimate of the true moisture content.
Nuclear radiation-based sensors are expensive and present potential hazards. Infrared sensors provide mainly surface moisture content. Dielectric-based methods are increasingly used for the development of cost-effective reliable moisture sensors which can be mounted on-line and provide in real time moisture content representative of the whole volume of material (Nyfors and Vainikainen, Industrial Microwave Sensors, Artech House, 1989; Kraszewski, Ed., Microwave Aquametryxe2x80x94Electromagnetic Interaction with Water-Containing Materials, New York: IEEE Press, 1996; Baltes et al., Eds., Sensors Update, Volume 7, Wiley-VCH, 2000, Trabelsi et al., Electronics Letters, Volume 33 (10), 874-876, 1997; Trabelsi et al., Trans. ASAE, Volume 42 (2), 531-536, 1999). The dielectric properties of a given material are intrinsic properties represented by the relative complex permittivity which is a complex number, often written as ∈=∈xe2x80x2xe2x88x92j∈xe2x80x3. The real part, ∈xe2x80x2, reflects the ability of a material to store electric-field energy, and the imaginary part, ∈xe2x80x3, is usually associated with the ability of a material to dissipate electric energy in the form of heat. j={square root over (xe2x88x921)} is the imaginary unit. During the past few years a trend for using higher and higher frequencies developed with a shift from the frequency range 106 Hz-108 Hz to the frequency range 109 Hz-1011 Hz. The essence of microwave moisture sensing is based on the polar nature of the water molecules, which translates into a high correlation between the dielectric properties of moist substances and their moisture content (Hasted, Aqueous dielectrics, London: Chapman and Hall, 1973), the absence of ionic conductivity, and the spatial resolution of the electromagnetic waves, providing information relating to the whole volume of the material rather than limited to the surface of the material. For most wet dielectric materials (Hasted, Aqueous dielectrics, London: Chapman and Hall, 1973), both ∈xe2x80x2 and ∈xe2x80x3 are dependent on the wave frequency, temperature of the material, its water content, and to some extent its composition. For particulate and granular materials, they also depend on packing properties that produce bulk density variations (Nelson, Cereal Chem., Volume 58 (6), 487-492, 1981; J. Microwave Power, Volume 18 (2), 143-153, 1983). At a given frequency, temperature and bulk density have effects similar to that of moisture content on the dielectric properties; both ∈xe2x80x2 and ∈xe2x80x3 increase linearly with temperature, moisture content, and bulk density. Therefore, development of microwave moisture sensors based on the principle of dielectric properties measurements requires that effects related to variables other than moisture content be accounted for or eliminated by using calibration techniques that are not sensitive to those variables.
The compensation/correction approach implies combination and integration of several different sensing devices, such as sensors for bulk density and temperature, which always increase the overall cost and complicates the calibration procedures. Also, the multiple computing steps introduce a higher probability for errors in case of malfunction or failure of one or more of these sensing devices. All of these aspects complicate the design, implementation, and maintenance of a moisture sensor dedicated to routine real-time measurements of moisture content. The use of temperature- and density-insensitive calibration methods is more attractive technically and economically. This approach simplifies considerably the design, calibration, and maintenance of a moisture-sensing instrument. There have been no calibration methods that eliminate both the temperature and the density effect. Temperature can be measured with relatively inexpensive devices and its effect compensated for in the output of the sensor. However, bulk density is more troublesome, and its determination may require costly devices, particularly those designed for on-line applications. For these reasons, more academic and engineering efforts have been devoted to solving the bulk density fluctuation problem. As a result, a few density-independent calibration functions were proposed (Kraszewski et al., Journal of Microwave Power, Volume 12 (3), 241-252, 1977; Meyer and Schilz, IEEE Trans. Microwave Theory tech., Volume MTT-29 (7), 732-739, 1981; Kent and Kress-Rogers, Trans. Inst. M C, Volume 8, 161-168, 1986; Kupfer, In: Microwave Aquametry, Ed.: A. W. Kraszewski, Chapter 21, 313-327, New York, IEEE Press Book Series, 1996; and Menke et al., IEEE MTT-S International Microwave Symp. Digest, Volume 3, 1415-1418, 1996) and some were successfully used in the development of microwave moisture sensors (Nyfors and Vainikainen, 1989, supra; Kraszewski, 1996; supra; Baltes et al., 2000, supra). Though moisture content is determined independent of density, these calibration functions are either instrument-specific (Kraszewski, 1977, supra; Menke et al., 1996, supra) or require an individual calibration for each type of particulate and granular material (Kraszewski, 1977, supra; Meyer and Schilz, 1981, supra; Kent and Kress-Rogers, 1986, supra; Menke et al., 1996, supra). Therefore, there remains a need in the art for a universal calibration method that remains valid for different particulate and granular materials and that is transferable across instruments. The present invention provides a calibration method and apparatus which are different from the prior art and solves most of the limitations cited above.
The present invention is a dielectric-based calibration method that is density- and material-independent and that remains valid across instruments of different designs. It consists of a universal calibration method and apparatus that provide moisture content from a single moisture calibration equation, with temperature compensation, established from measurements at a single frequency of the dielectric properties of particulate and granular materials presenting significant structural (shape, dimensions, and surface characteristics) and compositional differences.
It is therefore an object of the present invention to provide a method and apparatus for determining moisture content in any particulate or granular material at any radio-frequency and temperature without knowledge of bulk density from a single moisture calibration equation.
A further object of the present invention is to provide a method and apparatus for determining moisture content in any particulate or granular material using a universal calibration algorithm that remains valid across instruments of different design.
Further objects and advantages of the invention will become apparent from the following detailed description.